This invention relates generally to radar systems, and more specifically to a radar system which is capable of synchronization with a digital elevation map (DEM) to accurately determine a location.
The proper navigation of an aircraft in all phases of its flight is based to a large extent upon the ability to determine the terrain and position over which the aircraft is passing. In this regard, radar systems, altimeters, and other instrumentation in combination with the use of accurate electronic terrain maps, aid in determining the flight path of the aircraft. Electronic terrain maps provide the height of objects on a map and are presently used to assist in the navigation of aircraft.
Pulse radar altimeters accurately determine altitude using leading edge return signal tracking. Specifically, a pulse radar altimeter transmits a pulse of radio frequency (RF) energy, and a return echo is received and tracked using a tracking system. The interval of time between signal bursts of a radar system is referred to as a pulse repetition interval (PRI). The frequency of bursts is referred to as a pulse repetition frequency (PRF) and is the reciprocal of PRI.
A radar altimeter mounted on an aircraft transmits a signal that impinges a ground patch bounded by an antenna beam. As is well known, the Doppler effect results in isodops based on selection by Doppler filters within the radar altimeter. The area between the isodops is referred to as swaths. The Doppler filter, and resulting isodops are well known in this area of technology and will not be explained in any further detail. To scan a particular area, range gates are used to further partition the swath created by the Doppler filter. To scan a certain Doppler swath, many radar range gates operate in parallel. With the range to each partitioned area determined, a record is generated representing the contour of the terrain below the flight path. The electronic maps are used with the contour recording to determine the aircraft""s position on the electronic map. This system is extremely complex with all the components involved as well as the number of multiple range gates that are required to cover a terrain area. As a result, the computations required for this system are very extensive.
In addition to the complexity, the precision and accuracy of the distance to a particular ground area or object has never been attained using an airborne radar processor.
In one aspect, a method for determining location of a radar target is provided. The method comprises determining an interferometric angle, "PHgr", to the radar target based on at least one radar return and filtering the interferometric angle, "PHgr", through an adjustment of the effect of terrain features contributing to the interferometric angle, "PHgr". The adjustment is proportional to a degree of radar return fading resulting from the terrain features of the radar target. A corrected interferometric angle, "PHgr"out, is then provided, based at least in part on the filtering.
In another aspect, an apparatus for filtering an interferometric angle to a radar target to counter effects of terrain return fading due to summation of out of phase returns in the calculation of the interferometric angle to the radar target is provided. The apparatus comprises at least one signal fade detector, each detector receiving a signal representative of a radar return for one of a plurality of radar channels and outputting a quality factor signal, Qfade, that represents a measure of a depth of the signal fade for that radar channel. The apparatus also comprises a filter which receives the interferometric angle, "PHgr"in(n), and the quality factor signals. The filter outputs a weighted interferometric angle, "PHgr"out(n), which is weighted according to the received quality factor signals.
In still another aspect, a unit for determining an interferometric angle to a radar target is provided. The unit receives radar return data from a right radar channel, a left radar channel, and an ambiguous radar channel, where the ambiguous radar channel has an antenna located between antennas for the left and right radar channels. The unit comprises a phase processor receiving the radar return data from the right, left, and ambiguous radar channels. The phase processor determines the phase between the left radar channel and ambiguous radar channel return data, the phase between the ambiguous radar channel and right radar channel return data, and the phase between the right radar channel and left radar channel return data. The unit also comprises a phase ambiguity processor receiving the determined phases from the phase processor, and calculating a preliminary interferometric angle, "PHgr"in(n), to the radar target. The unit further comprises a plurality of signal fade detectors and a filter. One detector receives right radar channel return data, a second detector receives left radar channel return data, and a third detector receives ambiguous radar channel return data. Each detector outputs a quality factor signal that represents a measure of a depth of the signal fade for that radar channel. The filter receives the preliminary interferometric angle, "PHgr"in(n), from the phase ambiguity processor and the quality factor signals from the signal fade detectors. The filter outputs a weighted interferometric angle, "PHgr"out(n), which is weighted according to the received quality factor signals.